This invention relates generally to hand-held power tools, more particularly to a hand-held pneumatically powered tool having an output shaft and a reversible ratchet wheel drive therefor.
The invention is especially concerned with a hand-held pneumatically powered tool primarily of the type disclosed in U.S. Pat. No. 5,535,646 dated Jul. 16, 1996 entitled Ratchet Drive, also of the type disclosed in the coassigned U.S. Pat. No. 6,158,528 dated Dec. 12, 2000, entitled Hand-Held Pneumatic Rotary Drive Device, and in the coassigned copending application of Apr. 20, 2000, Ser. No. 09/553,921, entitled Hand-Held Power Tool, all these being incorporated herein by reference.
Referring to said U.S. Pat. No. 5,535,646 (briefly the '646 patent), the tool disclosed therein is shown to include an arrangement of washers including a friction washer (numbered 36) and one or preferably two Belleville spring washers (numbered 38) which combination, as stated in lines 58-60 of column 3 of the specification of the patent, “serves to pre-load the drive gear axially and prevent oscillation thereof.” The primary purpose of this arrangement of washers is to inhibit uncalled-for rotation of the “drive gear” numbered 22 (the ratchet wheel), such as retrograde rotation thereof when the tool is set for driving the ratchet wheel in one or the other direction, as may be caused by back-drag of the pawl which has been selected for driving the ratchet wheel. Such retrograde rotation has the effect of detracting from the power of the tool. In this regard it may be observed that since in the use of the tool the ratchet wheel may be driven forward in one direction by the one pawl and forward in the other direction by the other pawl it is not possible to use means such as a back-check pawl for positively preventing retrogressive rotation of the ratchet wheel. While the '646 patent arrangement of washers has been generally satisfactory for the purpose, it has encountered the problem of relatively short life due to washer wear and the consequent necessity for shimming up the arrangement or replacing washers.
One attempt to solve the problem of rapid wear is shown in U.S. Pat. No. 4,722,252. The Belleville washer is replaced by coil springs received in holes in the ratchet wheel which bias ball bearings against a wear washer. An advantage is that the ball bearings can be of a hardness similar to that of the wear washer. However, because the ball bearings engage the wear washer substantially at a point, the ball bearings act like cutters as the tool operates and the ball bearings move in a circular path over the wear washer, damaging the wear washer and reducing service life. Still another solution may be found in U.S. Pat. No. 5,896,789, in which ball bearings are eliminated and springs move in a circular path to directly engage a non-rotating wear washer. To work, the ends of the springs must be machined flat, which adds cost to production. In addition, the surface area of engagement between the springs and washer is still very small. Moreover, it is difficult to economically use springs which are sufficiently hard for wear purposes, but retain the needed resiliency. A further solution may be found in published Operator Instructions for Blue-Point® ratchet wrenches, model numbers AT2000 and AT205A, published by Snap-on Tools Company, in which two springs are used with two pins that directly engage a thrust washer. As with the springs alone, the contact area of the pins is much smaller than the area of the thrust washer, which leads to excess wear of the thrust washer as the pins move in a circular path across its surface.